Mycobacterium tuberculosis, the causative agent of human tuberculosis, kills more people than any other single infectious agent. The prevalence of TB is greatest in the developing world but its control in the United States has become severely complicated by the appearance of multidrug resistant strains of M. tuberculosis. There has recently been a sharp increase in the incidence of these MDRTB strains in developing countries. Effective control of tuberculosis requires improved and more rapid diagnostic methods, more efficacious vaccines, and better antimycobacterial drugs, particularly for treatment of multidrug resistant infections. With the recent advances in mycobacterial genetics and the determination of the complete genome sequence of M. tuberculosis, there is now renewed hope that a more sophisticated understanding of the physiology, genetics, and metabolism of M. tuberculosis will lead to novel strategies for controlling mycobacterial infections. Unfortunately, in spite of these genetic tools and genomic information, we know little about the molecular basis of the fundamental aspects of mycobacterial physiology-such as slow growth, their unique cell wall, and DNA replication-let alone the molecular basis of mycobacterial pathogenesis. Viruses are powerful tools for genetic analysis of a broad range of organisms, and the viruses of mycobacteria (mycobacteriophages) are no exception. The use of mycobacteriophages was instrumental in the establishment of mycobacterial genetics and the creation of cloning vectors for the introduction of DNA into mycobacteria. More recently, recombinant reporter mycobacteriophages have been proposed as clinical tools for rapid determination of drug susceptibilities of clinical isolates of M. tuberculosis. This project aims at understanding the intimate interface between mycobacteriophages and their hosts. This interaction begins with the association of free phage particles with bacterial cells followed by injection of phage DNA into the cell. Phage DNA may then either integrate into the host genome and be genetically silenced, or reprogram the cell to direct it towards phage gene expression and subsequent cell lysis. By exploring these events we will gain insights into the regulation of gene expression, the structure of the mycobacterial envelope, and the process of phage-mediated cell lysis. We will also use proteomic approaches to understand the influence of phage gene expression on that of its host.